The term “plate” is used herein to refer generally to a flat sheet of metal with a substantially constant thickness, usually greater than 6 mm or one quarter inch. Plates can be joined by bolting and/or welding. Joining by welding often requires preparation prior to welding, called weld preparation. Weld preparation typically includes creating sloped, largely straight faces on the plate edges. This is historically performed after a cutting operation in a separate, manual or semi-manual grinding process, which typically includes a great amount of additional handling, delay, labor, and cost.
FIG. 1 shows a conventional straight cutting NC machine 100 that includes a controller 150, which drives a vertical plasma torch 125 for cutting individual parts from a plate 160 that rests on a flame cutting bed 140 supported by steel slats. Typically, a nest of multiple parts are cut from plate 160 in one operation. A torch lifter 130 moves plasma torch 125 only along a vertical axis. Gantry 135 moves plasma torch 125 in an X-Y plane. Machines, similar to NC machine 100, have been used for plasma torch weld preparation, but these machines are expensive, inaccurate and difficult to program. Conventionally, these machines perform weld preparation as a separate process from part cutting, thereby suffering from many of the problems experienced with manual or semi-manual weld preparation described above. Interaction between a bevel head, a cutting machine, a plate to be cut, a plasma torch, the gantry, and the torch lifter are all controlled by the NC controller 150, which coordinates and controls motion in all axes. Conventional NC controllers include a torch height control that is voltage driven, requiring separate electronics and a separate lifting device (e.g., lifter 130).
Typical weld preparations are V, Y, X, K, J, and U preparations, where such letter characters illustratively represent the shape of the weld preparation. J and U preparations are used for very thick plates. X and K preparations, also called “double-V,” are more common, since the cross-sectional area of such preparations are half of that required for a V preparation.
FIG. 2 illustrates a conventional weld preparation diagram useful for V, Y, X, and K preparations. The illustrated weld preparation can be made by one to three straight cuts with a plasma torch held at an angle. A supporting face 202 of a first part is welded to a weld face 201 (also known as a root face) of a second part that has been manually or semi-manually ground back to form grooves A1 and A3. Grooves A1 and A3 are formed at an angle to allow access to weld face 201. To allow for weld material between weld face 201 and supporting face 202, a root gap G is required. First, root gap G is welded, and then grooves A1 and A3 are welded. Failure to include the root gap in the part dimensions can result in an unusable part after the part is cut. The angle of root face 201 is known as the dihedral angle D. The dihedral angle D is determined by the relative angle of, for example, surface 210 and 201, irrespective of the type of weld face eventually created. The root face is also known as the “land,” and parameters Z3 (depth to the top of the land from surface 210), Z1 (thickness of the plate), Z2 (depth of the bottom of the land from surface 210), and R1 (height of the bottom of the land as measured from the bottom surface 220) are also shown.
Some parts require additional processing prior to welding, for example by pressing and/or rolling into a bent shape such as a pipe end. In such a situation dihedral D and grooves A1, A3 may vary continuously relative to the flat plate being cut. This type of continuously varying cut is referred to as a Varying Bevel Angle or VBA. To produce a VBA on a part that is cut from a flat plate requires the torch to be capable of both swivel and tilt, as described using a C-axis and an A-axis, respectively. FIG. 3 shows a tilt axis (A-axis) 320 with three torches 125A+, 125A−, 125A0, positioned at tilt angles equal to A+, A− and A0, respectively. FIG. 4 shows a three dimensional Cartesian co-ordinate system 400. In FIG. 4 tilt axis (A-axis) 320 is overlaid onto the Z-axis such that a tilt angle of A+ is measured from the vertical Z-axis. Tilt axis 320 of FIG. 4 defines a first canonical axis. FIG. 4 also shows the plane of the tilt 410 and the swivel as described by C+ measured from the C-axis, or swivel axis 420. Plane of the tilt 410 measured by angle ‘A’ from tilt axis 320 is a plane bounded by a main vector 430 and its projection 440 onto the XY plane. It is shown as the vertical right triangle in FIG. 4. The C-axis 420 is a second canonical axis, which in FIG. 4 is overlaid onto the X-axis.
FIG. 4 shows the A- and C-axes conventionally used in a tilt and spin system for an oxyacetylene cutter, which do not suffer from cable wrapping. However, oxyacetylene cutter systems are not functional in plasma cutter environments, which do suffer from cable wrap. One example of a plasma cutter system cut that causes cable wrap is a looping corner cut. A looping corner cut may wrap cables by 270 degrees, at which point the plasma cutter rotation must be stopped before continuing a cut.
One conventional oxyacetylene NC machine uses a three torch cutting tool to simultaneously cut up to three faces at a time, making possible V, Y, X, and K shaped cuts on the edge of a plate. However, by using three torches, additional machinery and a separate control for each torch are required. Thus, the triple-torch system adds significant weight and financial cost. Additionally, to avoid torch flame collision, the lead torch, center torch, and trailing torch have to be significantly offset from one another (typically, by at least 20 mm). Also, with all three torches cutting simultaneously, this arrangement limits internal and external corner cutting capability. Moreover, cutting a part with weld preparation from the middle of a plate requires first cutting a rectangular hole in the plate so that each of the three torches can be edge started in turn, thereby resulting in significant waste of plate material to make the additional holes.
Another type of plasma cutting tool is a plasma arc welder, which is powerful and fast, but leaves a bevel of no more than a 45-degree chamfer on the plate edge. One advantage of this type of plasma cutting over oxyacetylene cutting is that the plasma torch can melt material at 10,000 degrees in a focused stream. In comparison, oxyacetylene uses a combusting process that heats the plate being cut substantially, which makes multiple passes more difficult. In comparison, a plasma torch is faster, especially when cutting thinner materials. In addition, the heat transferred from the plasma torch to the cut part is far less as compared to an oxyacetylene cutting process, thereby reducing unavoidable movement produced by thermal expansion.
Most existing bevel head designs are subject to problems with cable twists when the bevel head swivels. Swiveling designs also cannot travel any great distance while maneuvering around corners before cables and hoses become twisted. This is problematic since the cables carry high voltage, high current, and hoses contain various explosive gases and cooling water. Even when this conventional cable twist is minimized, the continual twisting cause material fatigue within the cable sand hoses, which reduces the service life of both the cable and its sheathing, resulting in higher repair cost and increased machine down time.
Additionally, attempts to reduce the overall weight of the bevel head and the machine have also resulted in an undesirable loss of structural strength for the resulting machine. Loss of structural strength leads to additional difficulties in managing bending, oscillation, natural harmonics, fracture stress points, distortion, and bearing loads. Bending and vibration are of particular concern with conventional pantograph designs. Weight is a major design consideration when fitting a bevel head onto an existing machine as an upgrade. Many currently available bevel heads are too large and too heavy; for example, the ESAB VBA head weighs one ton. Even recent, light weight heads weigh over 100 kg, plus the weight of substantial bulky cables for connecting the bevel head to remote amplifiers.
Conventional pantograph mechanisms allow manufacture with lighter materials and reduce the problems associated with cable twist, but these conventional pantographs cannot work in a smooth, controlled, accurate, and predictable way when coupled to a servo feedback system. Thus, conventional pantographs have compatibility issues with servo control systems, particularly when the torch used is positioned at, or close to, vertical. That is, conventional pantographs perform feedback directly on the A-axis and the C-axis, which causes instabilities. One example of this is the instability caused by a vertical torch position (A=0) resulting in the C-axis value being indefinite.
Controlling the height of conventional torches, especially when the torch cuts at an angle, has been another problem in the prior art. A torch held at 45 degrees with a 1 mm error in torch height, for example, can results in a 1 mm error in the cut path. Where a profile is created by multiple passes, the combined path error from the height error can add up resulting in an unusable part. Furthermore, the extreme environment near the point of cutting prevents the use of most known height control techniques. Arc voltage is conventionally used to predict torch height for plasma cutting, but conventional arc voltage height controls on plasma have, at best, a +/−2 mm accuracy. This accuracy problem is exacerbated when material is removal in multiple passes, for example, creating multiple faces in multiple passes. Conventional devices therefore use separate electronics and controls for the Z axis, which adds substantially to the cost to and weight of the system.
In addition to the height limitations discussed above, conventional cutting devices are also limited in the angle at which the torch can be tilted during a cutting operation. Although some conventional high definition plasma torches can cut at up to a 55 degree angle in some directions, conventional torch holders are incapable of tilting the cutting torch towards the pantograph at an angle greater than 45 degrees. A problem with the 45 degree angle limitation can be seen with respect to FIG. 11. FIG. 11 illustrates the difficulty encountered when cutting a bevel around a sharp corner. Path 1102 represents the top of the plate and 1103 the bottom of the plate. Vectors 1110, 1112 represent the torch vector at 45 degrees to the vertical while preparing the sloped faces 1100, 1101. The objective is to reach sloped face 1101 so that faces 1100, 1101 remain at constant angle and join at vector 1111. The torch, therefore, when reaching point 1120 has to remain stationary in the XY plane while the azimuth (C) changes smoothly and the tilt (A) increases until the vector at 1111 is achieved. This process cannot be accomplished by a torch limited to a 45 degree tilt, and such weld preparations thus have to be performed manually.
Another difficulty in creating corner cuts is shown in FIG. 12. Conventional devices must maintain a constant tilt angle to create a sharp corner. As shown in FIG. 12, a cutting torch moves along a cut path 1212 past a corner 1205, but then the torch must be turned 270 degrees in a looping path 1202 to approach corner 1205 again, in order to create the corner for a part without changing the torch tilt angle. Turning the torch in looping path 1202, however, requires the torch to cut into a waste portion of the plate, thereby increasing the amount of waste material and material cost for each cut part. Other conventional techniques to create corners (i.e., corner 1206 of path 1214 or corner 1204 of path 1210) at constant bevel angles include the reverse loop (1203) and the triangular corner (1201), but these alternative techniques still result in similar waste issues as seen with the use of looping path 1202.